ABI: uint8 too small for potential `complex quad` itemsize bits?

If quad-precision takes off, then the current `uint8` for the itemsize cannot represent the `128+128 = 256` bits of a complex quad number. Even now that would fail for most `long double` storage formats (those cannot be represented right now, so I am not sure it matters).

If an ABI break is necessary in the future, maybe this should be bumped to uint16 (maybe some other ones as well)? I guess there will be other ways to work around the limitation, though. And I admit quad-precision complex may well be as bad as it gets and is _just_ at the limit.